The present invention is directed to a liquid developer composition especially suitable for generating two-color images. More specifically, the present invention is directed to a developer useful, for example, in a process wherein electrostatic latent images formed on the surface of an imaging member are developed in a single step with a liquid developer containing first and second toner particles of opposite polarities, wherein the first and second toner particles are of different colors.
Electrophotographic image formation wherein a two-color image is developed in a single step can be performed either with a dry developer or with a liquid developer. With a dry developer, the latent image may be developed in a single step with a developer composition wherein toner particles of opposite charge and different colors are present in a developer housing, and toner particles of each color are selectively attracted to different portions of the latent image to result in a two-color developed image. Development of two-color images according to this process and with a liquid developer functions in a similar manner. One problem, however, that arises with the liquid developers relates to the colloidal stability of the developer composition. When a liquid medium contains toner particles of two different colors and opposite polarities, the particles will tend to attract each other to form particle agglomerations or aggregates with no net charge. Since the agglomerations or aggregates are essentially neutral with respect to charge, development is difficult or substantially nonexistent, since the mutual attraction between the particles is greater than the attraction between the particles and the portions of the latent image with opposite polarity.
Accordingly, a major difficulty in formulating liquid developers suitable for developing two-color images in a single step or pass is the preparation of a colloidally stable developer composition, wherein the oppositely charged toner particles do not agglomerate or aggregate to an extent that renders development difficult or impossible, or that results in poor quality images. The developer compositions of the present invention are intended to overcome this difficulty. One embodiment of the present invention comprises an electrophotographic developer composition comprising a liquid medium, first toner particles charged to one polarity and comprising a resin and a first pigment of one color, second toner particles charged to a polarity opposite to that of the first toner particles and comprising a resin and a second pigment of a color different from that of the first pigment, and a charge director.
Methods of generating two-color electrophotographic images are known. For example, U.S. Pat. No. 4,264,185, the disclosure of which is totally incorporated herein by reference, discloses an apparatus for developing images of two different colors. The apparatus of this patent is used in a development process wherein an electrostatic latent image of two different polarities is created on the imaging member and dry toner particles of opposite polarities, which are kept in two separate housings, are applied to the bipolar latent image for development. Preferably, the two toners are applied sequentially; in all instances, the two toners must be kept separate to prevent them from attracting each other such that their opposite charges are neutralized and both toners become incapable of developing latent images.
Another reference, U.S. Pat. No. 4,500,616, the disclosure of which is totally incorporated herein by reference, also discloses a method for developing two-color images with dry toner. According to this method, images of both positive and negative polarities are generated on a two-layered imaging member by means of a multi-stylus electrode, followed by development with two toners of different colors and opposite polarity. These two toners are mixed together to form one complex developer composition, and each image is developed under a magnetic bias by a process wherein the toner of one polarity is selectively extracted from a second toner of opposite polarity in the presence of an alternating field. This patent is directed to an imaging method employing multiple pass development.
U.S. Pat. No. 4,524,117, the disclosure of which is totally incorporated herein by reference, also directed to a multiple pass development method, discloses a method for the formation of two-colored image simultaneously. The method comprises uniformly charging a photoreceptor having a photoconductive layer sensitive to a first color, exposing a two-colored original to form on the photoconductive layer a latent image corresponding to a second color region in the original with the same polarity as the electric charges on the surface of the photoconductive layer, subjecting the photoreceptor to reversal development treatment by the use of a photoconductive color toner charged with the same polarity as the electric charges constituting the latent image to develop the non-charged region with the photoconductive color toner, subjecting the latent image to a normal development treatment by the use of an insulative toner having a color different from the color of the photoconductive color toner, and charging the color toners on the photoconductive layer with a different polarity from the charging polarity and simultaneously exposing the original through a filter shielding against the first color, thereby forming a two-colored image corresponding to the original. Methods for developing two-color images from latent images of positive and negative polarities by exposing them to two toners of different color and opposite polarity are also disclosed in Japanese Patent No. 56-87061 and Japanese Patent No. 58-48065.
In addition, U.S. Pat. No. 3,013,890, the disclosure of which is totally incorporated herein by reference, discloses a method of producing two-color images in which a charge pattern is developed with a single, two-color dry developer. The developer comprises toner particles of two different colors and opposite polarities and a single carrier capable of supporting both positively charged toner particles and negatively charged toner particles. According to this method, positively charged areas are developed with the negative toner particles and negatively charged areas are developed with the positive toner particles. When the charge pattern includes both positive and negative polarities, a two-color image results. Further, U.S. Pat. No. 4,312,932, the disclosure of which is totally inorporated herein by reference, discloses a color dry developing composition which obtains color images utilizing a single pass xerographic imaging system. The composition comprises toner resin particles containing up to four pigments and a single carrier. Corona charging may be used as a method of charging.
Liquid electrophotographic developers are also known. For example, Netherlands Patent No. 6,919,431 discloses a liquid electrophotographic developer containing a plurality of first particles and a plurality of second particles suspended in a liquid carrier medium. The first particles are electrical insulators, while the second particles have a tendency to assume the polarity of the field of the image. The first particles also tend to adhere to the surface of the image, while the second particles tend to be repelled, which leads to uniform development and no depositing of developer in non-image areas.
German Patent No. 1,225,049 discloses a process for producing a liquid electrophotographic developer by dispersing two oppositely charged toners in a carrier liquid, characterized in that two oppositely charged toners are used and their particles agglomerate to result in a composite particle of reduced charge. In the composite particles thus formed, one part has a positive charge and the other part has a negative charge. The resultant charge depends on which part has the greater charge; in any case, the resultant charge on the composite particle is lower than the individual charges on the original particles. The process disclosed by this patent yields a developer from which a larger number of toner particles are deposited on the latent image than with developers not containing composite particles, which results in improved image density.
Japanese Patent No. 55-124156 discloses a method for developing two-color images with a liquid developer. The developer composition comprises two kinds of insulating liquids of different specific gravities that do not mix with or dissolve in each other, such that two separate phases exist in the solution. One toner is contained in the first liquid, and another toner of different color and opposite polarity with respect to the first toner is contained in a second liquid. Since the liquids maintain separate phases, the two toners of opposite polarities do not attract each other.
Another reference, U.S. Pat. No. 3,793,205, discloses a developer composition comprising an insulating carrier liquid, a developer pigment of one polarity, and a second developer medium of opposite polarity to the first. The second developer medium enhances the deposition of the first pigment onto the imaging areas by increasing its sensitivity and allowing it to be deposited more heavily. The second developer medium also shields non-imaging background areas from visible contamination.
British Patent Application No. 2,169,416A discloses a liquid developer composition comprising toner particles associated with a pigment dispersed in a nonpolar liquid, wherein the toner particles are formed with a plurality of fibers of tendrils from a thermoplastic polymer. This application also discloses a process for preparing the disclosed liquid developer. In addition, U.S. Pat. No. 4,476,210 discloses a liquid developer composition and a method of making the developer, which developer comprises a marking particle dispersed in an aliphatic dispersion medium, wherein the marking particle comprises a thermoplastic resin core having an amphipathic block or graft copolymeric steric stabilizer irreversibly chemically or physically anchored to the thermoplastic resin core, with the dye being imbibed in the resin core and being soluble therein and insoluble in the dispersion medium.
Copending application U.S. Ser. No. 197,130, now U.S. Pat. No. 4,830,945, the disclosure of which is totally incorporated herein by reference, also discloses a developer suitable for a process wherein electrostatic latent images formed on the surface of an imaging member are developed in a single step with a liquid developer containing a plurality of first toner particles and a plurality of second toner particles, wherein the first and second toner particles are of opposite polarities and different colors. The developer comprises a liquid medium, first toner particles charged to one polarity which comprise a first dye of one color and polymeric cores to which steric stabilizer polymers have been attached, second toner particles charged to a polarity opposite to that of the first toner particles which comprise a second dye of a color different from the color of the first dye and polymeric cores to which steric stabilizer polymers have been attached, and a charge director. Further, copending application U.S. Ser. No 147,131, the disclosure of which is totally incorporated herein by reference, discloses a process for generating two-color images, comprising charging an imaging member; creating on the member a latent image comprising three different levels of potential consisting of a high level of potential, an intermediate level of potential, and a low level of potential; providing an electrode having a potential within 100 volts of that of the intermediate level of potential such that an electric field is generated between the member and the electrode, thereby creating a development zone between the electrode and the imaging member; and developing the latent image by introducing into the development zone a liquid developer composition containing first toner particles of one color and second toner particles of another color, said particles being dispersed in a liquid medium, such that the second toner particles are attracted to the high level of potential and the first toner particles are attracted to the low level of potential, with the intermediate level of potential remaining undeveloped. Copending application Ser. No. 197,131 also discloses developer compositions suitable for use in the disclosed process.
The process of charging a photoresponsive imaging member to a single polarity and creating on it an image consisting of at least three different levels of potential of the same polarity is disclosed in U.S. Pat. No. 4,078,929, the disclosure of which is totally incorporated herein by reference. This patent discloses a method of creating two colored images by creating on an imaging surface a charge pattern including an area of first charge as a background area, a second area of greater voltage than the first area, and a third area of lesser voltage than the first area, with the second and third areas functioning as image areas. The charge pattern is developed in a first step with positively charged toner particles of a first color, and, in a subsequent development step, developed with negatively charged toner particles of a second color. Alternatively, charge patterns may be developed with a dry developer containing toners of two different colors in a single development step. According to the teachings of this patent, however, the images produced are of inferior quality compared to those developed in two successive development steps. Also of interest with respect to the tri-level development process in U.S. Pat. No. 4,686,163.
Latent images generated according to the process disclosed in U.S. Pat. No. 4,078,929, hereinafter referred to as tri-level images, usually cannot be developed by sequentially applying two distinct liquid developers of different color and opposite polarity to the latent images because of the nature of liquid developers. While dry toners usually acquire charge by contact with carrier beads of opposite charge, liquid toners generally acquire charge by interaction with ionizable components in the liquid. Accordingly, in dry toners, the countercharges are contained on the carrier particles and are held under control by mechanical forces, while in liquid toners the countercharges are molecularly dispersed in the liquid. Thus, when an electric field is applied to a dry developer, only the charged toner particles migrate, and the countercharges do not migrate to the latent image; when an electric field is applied to a liquid developer, however, both the charged toner particles and the countercharges dispersed in the liquid migrate under the field. When tri-level images are developed with a liquid developer, the charged toner particles develop the areas of one bias, the background areas of second bias remain undeveloped, and the countercharges contained within the liquid developer tend to neutralize the areas of the third bias. As a consequence, only a degraded image with reduced contrast potential remains to be developed by a second liquid developer containing toner particles charged oppositely to the first toner particles.
Accordingly, while the compositions and processes of the above patents are suitable for their intended purposes, a need continues to exist for improved liquid electrophotographic developers suitable for generating two-color electrophotographic images. There is also a need for liquid developers wherein first and second particles with different colors and opposite polarities are present in the same developer solution. In addition, there is a need for liquid electrophotographic developers capable of developing two-color electrophotographic images in a single step. Further, a need exists for liquid electrophotographic developers wherein first and second particles with different colors and opposite polarities are present in the same developer solution without resulting in agglomeration of the oppositely charged particles to a degree that renders subsequent development with and separation of the particles difficult or impossible. Also, a need continues to exist for liquid electrophotographic developers wherein first and second particles with different colors and opposite polarities are present in the same developer solution and wherein the same charge director is employed to charge both the positive and the negative particles.